This post is one in a series featuring the complete slate of advanced energy technologies outlined in the report This Is Advanced Energy.

Image courtesy of Ford Motor Company.

Diesel engines are compression-ignition engines, meaning that they work by compressing air in the cylinders to heat it beyond the auto- ignition temperature of diesel fuel. When fuel is injected, combustion occurs without an external ignition source (such as a spark plug). Due to the higher compression ratio used in diesel engines and the higher energy content of diesel fuel, diesel engines can achieve 35% higher fuel economy than gasoline engines. Clean diesel engines are quieter, more efficient, more reliable, and cleaner than older diesel vehicles, thanks to innovations such as allowing for higher fuel-air mixing prior to combustion and the addition of re-circulated exhaust gas to the intake air stream.In addition, electronic controls and sensors throughout the vehicle ensure that just enough fuel is injected exactly when it is needed, improving efficiency.Using ultra-low-sulfur diesel fuel and exhaust treatment mechanisms such as on-board particulate traps and catalytic converters, clean diesel vehicles can achieve tailpipe emissions comparable to gasoline vehicles.The revelation of a “defeat device” in Volkswagen diesel cars in 2015 has called into question the emissions performance of clean diesel cars, though there is currently no evidence that other manufacturers have used similar tactics to circumvent U.S. tailpipe emission standards.The use of this device by one manufacturer does not negate the significant improvements in recent years with diesel vehicles of all classes.

Over 90% of heavy-duty vehicles in the United States are powered by diesel, and of these approximately one- third are clean diesel vehicles. Indiana, Utah, Oklahoma, Texas, and Wyoming lead the nation with clean diesel vehicles accounting for over 40% of heavy-duty trucks in those states. Diesel is not as popular for passenger vehicles, accounting for less than 3% of the U.S. market; however, this is expected to grow to 7% by 2020.Numerous clean diesel light-duty and medium-duty vehicles are now available, including models by BMW, Jeep, Ford, GMC, and Chevrolet.Outside the major automobile manufacturers, companies like Bosch have contributed improvements in various components of clean diesel trucks, vans, busses, and cars, from electronic engine controls to web-enabled operating efficiency to high-pressure injection systems.In addition, clean diesel technology can be used in rail and marine propulsion. For example, General Electric has introduced clean diesel locomotive and marine engines.

Because clean diesel engines improve fuel efficiency, they can contribute to significant cost savings over the life of a vehicle; a 2013 study estimated the three to five year total cost of ownership for light-duty diesel vehicles was typically $2,000 to $6,000 lower than that of equivalent gasoline-fueled vehicles, even after accounting for the higher initial cost.While cost savings are beneficial for owners, fuel savings also promote national energy security. If just one-third of U.S. passenger cars had diesel engines, petroleum consumption would decrease by up to 1.4 million barrels per day, equal to the amount of oil imported from Saudi Arabia.